Invasive Shrub Research Articles

Closed-Canopy Tropical Forests of Hainan, (China) Are Resilient against Invasive Herbs and Shrubs

The successful establishment of many plants in tropical forests often depends on species-specific adaptations related to light availability and forest successional stage. Species that are present in early successional stages generally do not occur in later successional stages. In this study, we documented the diversity, distribution, and abundance of terrestrial invasive plants across the (sub)tropical island of Hainan, China, and tested the germination of specific invasive plants in forested environments. In 97 transects positioned randomly across the island, we found nine invasive herb and shrub species were present in all human-modified habitats but not in intact forest interiors. In separate forest-specific transects, we documented a sharp drop in the abundance of invasive plants >5 m into the forest. High numbers of invasive plant seeds germinated from the soil seed bank sampled at the forest edge, but very few seeds germinated from soil sampled any distance into the forest. Finally, in experiments with four focal invasive plant species, overall germination rates were low; and much lower in shaded sites compared to full gap sites. In conclusion, our findings demonstrated that invasive herbs and shrubs do not yet form a serious threat to native species in the closed-canopy forests of Hainan.

The Impact of Invasive Buckthorn on Ecosystem Services and its Potential for Bioenergy Production: A review

ABSTRACT Two species of invasive shrubs are causing extensive environmental harm across North America. Common buckthorn (Rhamnus cathartica L.) and glossy buckthorn (Frangula alnus P. Mill.), originally imported from Europe, have become naturalized in many different ecosystems and are causing negative effects on associated ecosystem services. These invasive plants often create dense, monotypic stands that out-compete native plant species for nutrients and light. Current control methods, including manual or mechanical removal, are expensive and are mainly limited to destroying the buckthorn on site. The high cost of removal may be a barrier for restoration. Economic valuation should be carried out in terms of ecosystems services to better understand the cost-benefit of removing buckthorn. A cost-effective solution for removing buckthorn should also be explored. In this review, the state of the knowledge on the effects of buckthorn and its cost associated with major ecosystem services were synthesized. Existing methods to harvest and transport buckthorn and potential new methods were compiled. Finally, a policy review of buckthorn was completed for states where buckthorn is most prevalent. Findings from this review will be important for applying an economic cost to buckthorn and revealing where more work can be done.

Using Floristic Analysis and Dendrochronology to Plan Ecological Restoration within a Forest–Prairie Ecotone

The limestone bluffs of the central Mississippi River Valley represent a significant portion of the remaining intermingled xeric hill prairie-savanna-woodland systems of the Midwest where different cover types may co-occur at very fine scales. Efforts to restore these landscapes would benefit from a data-driven tool to delineate existing vegetation cover characteristics and reconcile these with desired management outcomes on a spatially explicit basis. This study employed hierarchical cluster analysis and nonmetric multidimensional scaling in combination with indicator species analysis and dendrochronological methods to examine the structure, composition, and temporal community patterns of the forest–prairie gradient in the Mississippi River Bluffs of Jersey County, Illinois. Four distinct community types were identified on the gradient: (1) a woodland community dominated by Juniperus virginiana, Carya glabra, Quercus stellata, and Lonicera maackii; (2) open oak woodland with high exposed soil and overstory of Acer saccharum, Q. velutina, and Q. muehlenbergii; (3) a hill prairie community with grass coverage as the sole indicator; and (4) a transitioning community with 14 significant indicators and a trajectory advancing toward mesophytic forest composition. These data, within context with stem age, historical documents, and drought data, reveal the role of historical disturbances in temporal patterns of forest–prairie dynamics and provide a resolution in greater detail than previous studies of forest and nonnative invasive shrub encroachment. Pairing management activities to stand conditions, as informed by understanding drivers of ecosystem change, provides continuity with historical disturbance regimes and an objective basis for restoration activities and targets.

Association of flower color with pollen reward may explain increased bumblebee visitation to the Scotch broom yellow morph

Given that pollinators usually visit flowers for hidden rewards, they need to rely on floral traits that indicate reward status (“honest signals”). However, the relationship between pollination, honest signals, and floral rewards is little documented in natural conditions. The Scotch broom (Cytisus scoparius) is an invasive shrub with polymorphism in the color of its flowers that can be yellow, orange, or red. In three areas dominated by the Scotch broom, we described the abundance of the floral morphs and estimated bumblebee (Bombus terrestris) visitation rate. We examined whether bumblebee visitation to the floral morphs was related to pollen reward. We collected flowers and classified their stamens according to their function: reward or pollen export. Then, we measured anther size and estimated pollen quantity. The yellow morph was more abundant and more visited by bumblebees than the orange and red morphs. The yellow flowers did indeed offer more pollen than the other morphs and this occurred only for rewarding anthers, suggesting that bumblebees could use yellow color as an honest signal to visit the most rewarding flowers. We discuss whether innate and/or learned preferences of bumblebees can explain why the yellow morph is more visited, pollinated, and abundant, while the other morphs are maintained at a lower frequency. This is one of the few field works that shows that variation in intra-specific floral traits is associated with variation in floral reward and pollinator visitation rate, helping to understand the foraging preferences of pollinators and the coexistence of floral morphs in nature. Clinical trials registration: Not applicable.

Landscape-scale hydrologic response of plant invasion relative to native vegetation in urban forests

Large-scale invasion modifies watershed hydrology by changing surface runoff and lowering the seasonal availability of water to native plants. Due to costly field-based evapotranspiration (ET) measurements, which are highly localized and occasionally subject to instrument failure, landscape-scale water use assessments of invasive plants are infrequent. Therefore, the extent to which plant invaders alter water allocation between native and non-native vegetation in a given landscape is rarely assessed. We used a remote sensing-based ET modeling approach to measure the hydrologic response of an invasive shrub, Ligustrum sinense, across forests of the Charlotte Metropolitan Area, North Carolina. We hypothesized that this invader's widespread occurrence and dominant plant physiology significantly competes with native forests for water resources. We tested this hypothesis by comparing inter- and intra-annual variations in ET from invaded and uninvaded sites estimated using the surface-energy-balance system (SEBS) model and cloud-free Landsat images for the wettest (2003), driest (2007), and normal (2005 and 2011) water years. Our findings suggest that the water demand of L. sinense is higher than native forests (deciduous and evergreen) for most of the year except during the early spring and after high precipitation events. The daily ET flux of L. sinense was significantly different than evergreen vegetation during the driest year (2007) that, five years later (2011 - normal water year), was significantly different than both deciduous and evergreen vegetation types. This suggests that L. sinense consumes more water than native forest types, particularly during dry and normal precipitation years with increasing canopy cover over time making it a strong competitor with native vegetation for water resources in urban forests. Therefore, accounting for the hydrologic response of invasive plants and potential water savings from their removal from forests, particularly in water-scarce regions, may enable land managers and decision-makers to prioritize areas for monitoring and control efforts.

Ecosystem functions in degraded riparian forests of southeastern Kenya.

Species community structures shape ecosystem functions, which are mostly stronger pronounced in intact than in degraded environments. Riparian forests in semiarid Africa provide important habitats for endangered plant and animal species and provide various ecosystem functions, that is, services to people settling along these streams. Most of these riparian forests are severely disturbed by human activities and dominated by invasive exotic plant species in the meanwhile. Thus, ecosystem functions are negatively influenced. While most studies have analyzed a specific metric to measure the degree of ecosystem function, little is known about how strongly different ecosystem functions respond to anthropogenic disturbances in parallel. In this study, we analyzed a set of four proxies of ecosystem functions, ground‐dwelling arthropod abundances, pollination, seed dispersal, and predation, along a highly disturbed riparian forest in southeastern Kenya. We assessed the land cover and land use manually and with an Unmanned Aerial Vehicle. Our data show that ecosystem functions respond differently to vegetation cover, human disturbances, and the availability of the invasive exotic shrub Lantana camara. The occurrence of representatives from the groups Saltatoria and Formicidae profits from heterogeneous habitat structures and natural riparian forest, while representatives of the Araneae profit from high proportion of agricultural fields. In general, predation is higher in mixed land use and natural riparian forest, while pollination and seed dispersal showed no significant trend in regard on land coverage. Along with this, predation also increased with rising proportion of natural riparian forest, while the proportion of agricultural land negatively affects predation, but in parallel showed a slightly significant positive trend with seed dispersal. Human disturbances and the occurrence of the invasive exotic L. camara shrub did not significantly affect our metrics of ecosystem functioning, except of the negative impact of human disturbances on pollinators. In conclusion, our results underpin that ecosystem functions respond highly variable and individually to environmental changes.

Plant diversity and community composition in managed humid coastal dune slacks in NW England.

Humid coastal dune slacks are an endangered habitat in Northwestern Europe. In the UK, dune slacks are currently classified as being in ‘unfavourable’ condition, with projected decrease in England of up to 30% by 2036. Studies in mainland Europe suggest that regional factors (e.g. slack area, age, and isolation) are more important than local factors (e.g. soil, pH, soil nutrient and water status) in driving successional vegetation processes in coastal slacks. However, this has never been tested for the UK, where approximately 14% of European slacks occur. We used previously established survey protocols to test whether regional factors are more important than local factors in UK coastal slacks, along the Sefton Coast in NW England. We found that slack area and slack age were more important than local factors in driving plant community composition and species richness. We also showed that higher levels of management, such as active grazing and invasive shrub and tree removal, are effective in increasing soil moisture levels in slacks. Our results suggest that similar successional processes are likely to be important in slacks in the NW of England, compared to mainland Europe.

Biology, distribution and management of the invasive Jatropha gossypiifolia (Bellyache bush): A global review of current and future management challenges and research gaps

AbstractJatropha gossypiifolia (Bellyache bush) is an evergreen, perennial, invasive woody shrub that globally threatens agricultural and native landscapes. The species is native to North and South America, but is currently found in over 52 countries; in many of these countries, it is listed as invasive, due to its aggressive and competitive growth. These behaviour traits are characterised by its quick flowering and seed production that make the species difficult to contain and control, as it forms dense monocultures and restricts the growth of many native and pastoral species through aggressive competition for resources, such as soil moisture and nutrients. Infestation by J. gossypiifolia results in a loss of biodiversity in the affected area and disrupts the maintenance of sustainable land systems. It has been reported that J. gossypiifolia is used in many countries as a traditional medicine to treat a range of diseases, illnesses and injuries. However, despite these claims of its positive use, J. gossypiifolia leaves, seeds and stems are highly toxic to grazing animals and humans, leading to disruption and significant increases in the cost of agricultural production through the loss of production and the instituting of necessary management efforts. Around the world, there have been various attempts to control J. gossypiifolia, but these have widely varied in their success. The most common and successful treatments to control J. gossypiifolia, including herbicide application (cut stump application), fire management and mechanical control; however, when each is applied as a single treatment, long‐term success is often limited. Consequently, this review explores the biology and distribution of J. gossypiifolia, together with previous and current control attempts, and synthesises potentially useful approaches to control this noxious weed, whilst identifying areas of future research to improve effective and long‐term sustainable management strategies for this noxious species.

The Seasonal Influence of Invasive Shrubs on Light and Temperature in an Eastern Deciduous Forest Understory

Invasive shrubs are flourishing in temperate, deciduous forest understories of eastern North America where resources, especially light, are limited. However, understory light is more available before the overstory canopy leaves emerge in the spring and after fall senescence. Extended leaf phenology of invasive shrubs in the spring and fall compared to native shrubs and the overstory canopy is conspicuous, as well as higher foliage abundance of invasive compared to native shrubs. Extended leaf phenology of invasive shrubs provides photosynthetic benefits, but also seasonally novel shade. Light and temperature regulate life history characteristics across taxa and influence ecosystem processes. Here, a long-term invasive shrub removal experiment is used to quantify the effect on light and air temperature over 3–4 y. There was a pattern of reduced maximum air temperature in the presence of invasive shrubs during the growing season of most years. We find less light energy infiltration (lumens m–2) below invasive shrubs than native shrubs with mean differences largest in the spring (–1981 [–2604, –1380], a 26.8% reduction relative to below natives). Differences diminish through summer (–1038 [–1221, –845]), fall (–547 [–660, –429]), and winter (–257 [–372, –151]). Invasive shrubs also filter more photosynthetically active radiation than natives (58.4 [38.5, 78.7] µmol m–2 s–1, a 39.4% reduction), but seasonal differences were not detected indicating denser canopy structure for invasive shrubs throughout the year. When compared to the native understory community, the presence of invasive shrubs seasonally reduces temperature and light availability near the forest floor, which could affect resident plant and animal species.

Can Sentinel-2 be used to detect invasive alien trees and shrubs in Savanna and Grassland Biomes?

The impact of alien plant invasions on ecosystems translates into major economic losses for nations across the globe. Freely available satellite imagery can be used to provide relevant data for effective management of alien plant invasions, especially for developing nations with limited budgets. However, detecting alien herbaceous and shrubby vegetation using freely available imagery has not been widely tested. The aim of this study was to detect invasive alien plants in the Savanna and Grassland Biomes of the uMngeni Catchment. The novelty of this study is the focus on not only alien trees, but also alien shrubs, which tend to be more sparsely spread and are harder to detect. Using the free Google Earth Engine Platform for repeatability, we explore the efficacy of data fusion of Sentinel-2, Sentinel-1 and landform data, various feature selection techniques and two powerful non-parametric classifiers. We found that feature selection techniques result in lower classification accuracies and are therefore not recommended for similar applications. Data fusion, such as combining Sentinel-2, Sentinel-1 and landform data, did not improve classification results. Support Vector Machine shows superiority over the Random Forest classifier. The final classifications indicate, with 80–90 % accuracy, that for 2019 about 17.5 % (736.15–763.75 km2) of the uMngeni Catchment was occupied by invasive alien trees and shrubs (e.g., gums, wattles, pines and Bugweed), either in the form of plantations or invasions. This translates to between 88.3 million and 173 million m3 of water per year in evapotranspiration losses, or 23 % or the current system yield. At a spatial resolution of 20 × 20 m, these outputs are useful for regional or local planning and prioritization. Furthermore, the repeatable approach of this study is a significant advance compared to past efforts that have produced publicly available datasets for the region, and could be employed by other data-scarce developing nations.

Episodic defoliation rapidly reduces starch but not soluble sugars in an invasive shrub, Tamarix spp.

Plants rely on pools of internal nonstructural carbohydrates (NSCs: soluble sugars plus starch) to support metabolism, growth, and regrowth of tissues damaged from disturbance such as foliage herbivory. However, impacts of foliage herbivory on the quantity and composition of NSC pools in long-lived woody plants are currently unclear. We implemented a controlled defoliation experiment on mature Tamarix spp.-a dominant riparian woody shrub/tree that has evolved with intense herbivory pressure-to test two interrelated hypotheses: (1) Repeated defoliation disproportionately impacts aboveground versus belowground NSC storage. (2) Defoliation disproportionately impacts starch versus soluble sugar storage. Hypotheses were tested by transplanting six Tamarix seedlings into each of eight cylinder mesocosms (2 m diameter, 1 m in depth). After 2.5 years, plants in four of the eight mesocosms were mechanically defoliated repeatedly over a single growing season, and all plants were harvested in the following spring. Defoliation had no impact on either above- or belowground soluble sugar pools. However, starch in defoliated plants dropped to 55% and 26% in stems and roots, respectively, relative to control plants, resulting in an over 2-fold higher soluble sugar to starch ratio in defoliated plants. The results suggest that defoliation occurring over a single growing season does not impact immediate plant functions such as osmoregulation, but depleted starch could limit future fitness, particularly where defoliation occurs over multiple years. These results improve our understanding of how woody plants cope with episodic defoliation caused by foliage herbivory and other disturbances.

Abstracts

Abstracts

Nonlinear plant-plant interactions modulate impact of extreme drought and recovery on a Mediterranean ecosystem.

Interaction effects of different stressors, such as extreme drought and plant invasion, can have detrimental effects on ecosystem functioning and recovery after drought. With ongoing climate change and increasing plant invasion, there is an urgent need to predict the short- and long-term interaction impacts of these stressors on ecosystems. We established a combined precipitation exclusion and shrub invasion (Cistus ladanifer) experiment in a Mediterranean cork oak (Quercus suber) ecosystem with four treatments: (1) Q.suber control; (2) Q.suber with rain exclusion; (3) Q.suber invaded by shrubs; and (4) Q.suber with rain exclusion and shrub invasion. As key parameter, we continuously measured ecosystem water fluxes. In an average precipitation year, the interaction effects of both stressors were neutral. However, the combination of imposed drought and shrub invasion led to amplifying interaction effects during an extreme drought by strongly reducing tree transpiration. Contrarily, the imposed drought reduced the competitiveness of the shrubs in the following recovery period, which buffered the negative effects of shrub invasion on Q.suber. Our results demonstrate the highly dynamic and nonlinear effects of interacting stressors on ecosystems and urges for further investigations on biotic interactions in a context of climate change pressures.

Biology, distribution and control of the invasive speciesUlex europaeus(Gorse): A global synthesis of current and future management challenges and research gaps

AbstractUlex europaeus(Gorse) is one of the most invasive shrubs in the world, being now found in more than 50 countries where it economically and environmentally degrades the land. This highly versatile shrub can live more than 30 years and produce over 18,000 fertile seeds annually that can remain viable for over 30 years.Ulex europaeusspread is facilitated by its ability to germinate in a wide range of conditions, quick growth and maturing rate and several seed dispersal mechanisms. Despite extensive research and attempts at managingU. europaeus, current rates of control are not adequate to limit the species’ competitiveness and impact on the environment. This has resulted in the species altering soil and landscape dynamics in areas of major invasion, inhibiting the growth of agricultural and native species, creating shelter for pest species and reducing the richness of competing species at a site, all of which contribute to the economic and environmental degradation of the land. This review highlights that herbicide application is the most successful technique used to controlU. europaeus, although it has shown varying success across different climatic regions. In this regard, future research should investigate the possibility of integrating a range of techniques (competition, fire management, grazing, manual removal and mycoherbicides) to increase control success and reduce any potential risk of herbicide resistance. Further research is also required on the maintenance of longer‐term viable populations of biological control agents to limit the success of this invasive species. Bridging the identified research gaps will help to facilitate the improved long‐term management ofU. europaeusand help land managers confidently to maintain sustainable land systems.

Controlling invasive alien shrub species, enhancing biodiversity and mitigating flood risk: A win–win–win situation in grazed floodplain plantations

The high nature conservation value of floodplain ecosystems is severely threatened by invasive alien species. Besides adversely affecting native biodiversity, these species also pose a major threat from a wider socio-ecological perspective (e.g. ‘roughness’ increases flood risk). Finding options to control dense shrub layers consisting of invasive alien species is therefore of high priority for multipurpose management. We studied cattle grazing impacts on the cover, composition and diversity of the herb and shrub layers in floodplain poplar plantations along the Tamiš river, Serbia. Non-grazed, moderately grazed, intensively grazed and resting place stands were sampled in five locations in three sampling points. Non-grazed stands had substantially higher cover of invasive alien shrub species (on average 65%) than moderately and intensively grazed stands, and resting places (5.17, 0.02 and 0.00%, respectively), but without considerable differences between the grazing intensity categories. The number of invasive alien species in the shrub layer decreased considerably from non-grazed to intensively grazed stands. Species composition in the herb layer changed from non-grazed to intensively grazed stands, while resting places differed substantially from the other categories. Total species richness, richness of native generalist herbaceous grassland species, and the cover of palatable grasses were the highest in moderately and intensively grazed stands. Our results suggest that cattle grazing in floodplains is effective at controlling invasive alien shrub species. Furthermore, continuous moderate or intensive grazing would contribute to multifunctional management of invaded floodplains by enhancing local biodiversity, reducing flood risk, and providing additional grazing areas for the local community.

Fleshy‐fruited invasive shrubs indirectly increase native tree seed dispersal

Biological invasions are one of the main threats to biodiversity and ecosystem functioning in the Anthropocene. Fleshy‐fruited invasive plants establish mutualistic interactions with native seed dispersers and may affect the mutualisms between native partners, negatively by disrupting plant dispersal through competition for seed dispersers or positively by facilitating seed dispersal via the attraction of fruit‐eating animals. Moreover, the invaders' density in the neighborhood of native plants may modulate the direction and/or magnitude of such effects on natives. In mountains of central Argentina, the cessation of fruiting of the dominant native tree Lithraea molleoides overlaps with the beginning of the fruiting of invasive shrubs (Pyracantha spp.). This partial overlap allows for testing opposite predictions regarding the effect of invasive fruits on native seed dispersal. We compared frugivory by seed disperser birds on L. molleoides during non‐overlapping and overlapping periods, at six sites that differed in Pyracantha shrub density (high, low and no‐invasion). We counted frugivory during 2 h on 15 individuals of L. molleoides at each site and period, totaling 360 h of observation. Frugivory on the native tree was similar among sites during the non‐overlapping period and remained constant in both periods at non‐invaded sites. At invaded sites, frugivory increased from non‐overlapping to overlapping period and was greater at highly invaded sites. The resource provided by Pyracantha did not disrupt seed dispersal of the dominant native tree. Rather, it facilitated frugivory by seed dispersers and this effect may be exacerbated with higher fruit availability of invasives. Our results provide a counterview to the prevailingly negative impacts of invasive species on the seed dispersal of native species since the fruiting of invasive plants facilitated the seed dispersal of native species in a continental region contrasts with the mutualism disruption commonly observed in insular environments.

Soil texture and other site-level factors differentially affect growth of Scotch broom (Cytisus scoparius) and Douglas-fir (Pseudotsuga menziesii) seedlings in the western Pacific Northwest

The invasive shrub Scotch broom (Cytisus scoparius (L.) Link) is a pervasive threat to regenerating Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) stands in the Pacific Northwest, USA. Field observations indicate that the susceptibility of areas to Scotch broom invasion and dominance can vary by site. We selected 10 sites throughout the western Pacific Northwest that spanned a gradient of soil textures and other factors to test the site-specific susceptibility of Douglas-fir to overtopping by Scotch broom. We expected to find that the ability of Scotch broom to dominate a site was mediated by site-level factors, particularly those influencing soil water — the most limiting factor to growth in the region. We found Scotch broom and Douglas-fir were inversely affected by site-level factors. In general, Douglas-fir absolute height growth rates were more competitive with those of Scotch broom on fine-textured soils than on more coarsely textured soils. We also found Douglas-fir to have a more dramatic response to increasing down woody material than Scotch broom. Scotch broom height growth approached an asymptote at 3 m. Sites with fast-growing Douglas-fir were able to surpass this height 6–7 years after planting and appear likely to avoid suppression by Scotch broom.

Invasive species in the Anthropocene: Help or hindrance?

Under predicted climate change scenarios many parts of the world will be hotter. Higher temperature extremes present significant physiological challenges to ectothermic freshwater species that cannot regulate body temperature. Willows (Salix spp.) are highly invasive deciduous northern hemisphere shrubs and trees that have colonised riparian zones of southern hemisphere streams. Non–native willows are criticised for their high consumption of water and their capacity to form dense monostands along the margins and within waterways that limit light to streams in summer, alter the timing and quality of allochthonous inputs and modify ecosystem function. As such, governments invest heavily in the removal of willows from streams in order to preserve ecosystem integrity. Although detrimental effects of non–native willows are well documented, little attention has been focussed on consideration of potential ecosystem services that non–native willow infestation may provide under predicted climate warming. Here, we use a case study to illustrate that shading by non–native willows can provide thermal refugia for temperature sensitive endemic taxa and we provide a holistic approach to non–native willow removal that may provide benefits to aquatic species amid changing climate. We present a simple decision matrix for prioritising willow removal activities that may be applied to other invasive species and we discuss traditional views of invasive species management and river restoration and their relevance in a rapidly warming world. The concepts we discuss are of immediate relevance to environmental managers challenged with maintaining and restoring ecosystems that are rapidly changing in structure and function in response to climate warming.

Impacts of invasive Amur honeysuckle, Lonicera maackii, leaf litter on multiple trophic levels of detritus‐based experimental wetlands

Abstract Wetlands are especially vulnerable to invasive plants because seasonal movements of sediments, water, nutrients, and debris from adjacent terrestrial habitats create ecological conditions suitable for invasion. Lonicera maackii is a relatively abundant and broadly distributed invasive shrub in the eastern U.S.A., yet few studies have simultaneously tested for effects of its leaf litter on multiple trophic levels within wetland food webs. Hatching success, hatchling survival, and hatchling size of tadpoles (Acris crepitans) were assessed using a laboratory mesocosm experiment with no‐leaf, native‐leaf, and L. maackii‐leaf treatments. In the field, short‐term (c. 3 week) and long‐term (c. 2 years) mesocosm experiments assessed the initial and persistent impacts, respectively, of L. maackii on measures of primary productivity, invertebrate abundance and community structure, and oviposition preference and larval survival of Cope's gray treefrog (Hyla chrysoscelis). Exposure of eggs to L. maackii leaves reduced body size at hatching and larval survival of newly hatched A. crepitans. In short‐term experiments, L. maackii leaves reduced dissolved oxygen levels, filamentous algal biomass, and macroinvertebrate abundance, and altered macroinvertebrate community structure. In long‐term experiments, duckweed cover was 15× times greater in mesocosms with L. maackii leaf additions. Oviposition by a local population of H. chrysoscelis was 10× lower in both short‐ and long‐term mesocosms with L. maackii present. Although ecological impacts of L. maackii have been reported for terrestrial systems, our study is the first to demonstrate the simultaneous effects of its leaves on multiple trophic levels within replicated experimental wetlands. The relatively rapid decomposition of L. maackii leaves, and associated pulse of phenolic compounds, is the most likely proximate mechanism of the bottom‐up effects we observed. The mechanisms, timing, and significance of effects is predicted to vary among natural wetlands. Our study demonstrates multiple ecological impacts of a terrestrial invasive shrub within experimental wetlands. Detailed studies on the specific mechanisms, and their spatial and temporal variability in natural systems, will elucidate management strategies and improve the efficiency of wetland conservation efforts.

Extending coverage and thematic resolution of compositional land cover maps in a hierarchical Bayesian framework.

Ecological models are constrained by the availability of high-quality data at biologically appropriate resolutions and extents. Modeling a species' affinity or aversion with a particular land cover class requires data detailing that class across the full study area. Data sets with detailed legends (i.e., high thematic resolution) and/or high accuracy often sacrifice geographic extent, while large-area data sets often compromise on the number of classes and local accuracy. Consequently, ecologists must often restrict their study extent to match that of the more precise data set, or ignore potentially key land cover associations to study a larger area. We introduce a hierarchical Bayesian model to capitalize on the thematic resolution and accuracy of a regional land cover data set, and on the geographic breadth of a large area land cover data set. For the full extent (i.e., beyond the regional data set), the model predicts systematic discrepancies of the large-area data set with the regional data set, and divides an aggregated class into two more specific classes detailed by the regional data set. We illustrate the application of our model for mapping eastern white pine (Pinus strobus) forests, an important timber species that also provides habitat for an invasive shrub in the northeastern United States. We use the National Land Cover Database (NLCD), which covers the full study area but includes only generalized forest classes, and the NH GRANIT land cover data set, which maps White Pine Forest and has high accuracy, but only exists within New Hampshire. We evaluate the model at coarse (20km2 ) and fine (2km2 ) resolutions, with and without spatial random effects. The hierarchical model produced improved maps of compositional land cover for the full extent, reducing inaccuracy relative to NLCD while partitioning a White Pine Forest class out of the Evergreen Forest class. Accuracy was higher with spatial random effects and at the coarse resolution. All models improved upon simply partitioning Evergreen Forest in NLCD based on the predicted distribution of white pine. This flexible statistical method helps ecologists leverage localized mapping efforts to expand models of species distributions, population dynamics, and management strategies beyond the political boundaries that frequently delineate land cover data sets.